Wahoo makes aluminum decking, which can be placed over areas that are meant to stay dry.
Here's an article they wrote on rust and aluminum decks.
Will an Aluminum Deck Rust?
It’s a question that we get asked often here at Wahoo Decks and it’s a relevant question. Many fear that because aluminum is metal, like most metal products, it will eventually rust. Even so, the fact of the matter is that aluminum does not rust; however, aluminum can corrode. First, let’s address how rust can actually affect aluminum and what you can do to prevent it.
“When iron-containing metals combine with oxygen, oxidation, and eventually rusting, occurs. Because aluminum, like copper, is not an iron-containing metal, it doesn’t actually rust itself. If you have a rust stain on an aluminum surface, it’s not because the aluminum itself has rusted. More likely, it has been stained from a nearby metal. If an iron-containing metal, such as steel, rest on, is attached to, or drips water on another surface, it can create a reddish-brown stain and make it appear that the surface has rusted” (Rust on Aluminum).